Powder metal internal gear rolling process

ABSTRACT

A method for forming an internal gear is provided comprising providing a powder metal preform workpiece, supporting the workpiece about its periphery, and roll forming said workpiece to produce a set of densified gear teeth of a predefined dimension.

REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. provisional application Ser. No. 60/917,373 filed May 11, 2007.

TECHNICAL FIELD

The following relates generally to powder metallurgy and has particular utility in forming internal gears from powder metal preforms.

BACKGROUND OF THE INVENTION

Internal gears are used in a broad range of mechanical devices in applications such as domestic appliances, automotive drive trains, agricultural and industrial machinery. The internal gears are traditionally manufactured using metal cutting techniques such as broaching or shaping. The cut steel gears are functional but are typically considered costly to manufacture due to the material that is discarded in the tooth cutting process.

Representative of the art is U.S. Pat. No. 5,711,187 (1998) which discloses a gear wheel formed from a pressed and sintered powder metal blank in which the metal powder comprises an admixture of iron powder and at least one alloying addition and the gear wheel is surface hardened by densifying at least the tooth root and flank regions to establish densification in the range of 90 to 100 percent of full theoretical density to a depth of at least 380 and up to about 1,000 microns.

Reference is also made to co-pending application Ser. No. 11/813,400 filed Dec. 23, 2005 which relates to a method of forming powder metal components having surface densification.

It is therefore an object of the following to provide a method for forming an internal gear is provided comprising providing a powder metal workpiece and roll forming the workpiece to produce a set of densified gear teeth of a predefined dimension.

SUMMARY

In one aspect, a method for forming an internal gear is provided comprising providing a powder metal workpiece and roll forming the workpiece to produce a set of densified gear teeth of a predefined dimension.

The invention comprises a method for forming an internal gear is provided comprising providing a powder metal preform workpiece, supporting the workpiece about its periphery, and roll forming said workpiece to produce a set of densified gear teeth of a predefined dimension

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of example only with reference to the appended drawings wherein:

FIG. 1( a) shows an internal gear rolling tool set.

FIG. 1( b) is a sectional view along the line A-A in FIG. 1( a).

FIG. 1( c) is a perspective view of the internal gear rolling tool set in FIG. 1( a).

FIG. 2 is an enlarged view showing material displacement during roll forming using the tool set of FIG. 1( a).

FIG. 3 shows surface densification after rolling.

FIG. 4 is a graph illustrating density versus distance from the surface of the workpiece for the tool set in FIG. 1( a).

FIG. 5( a) is an embodiment of a multiple rolling tool set.

FIG. 5( b) is a sectional view along the line B-B in FIG. 5( a).

FIG. 5( c) is a perspective view of the multiple rolling tool set in FIG. 5( a).

FIG. 6 is a perspective view of the multiple rolling tool set in FIG. 5( a).

FIG. 7 is an enlarged view showing material displacement during roll forming using the tool set of FIG. 1( a).

FIG. 8 is a graph illustrating density versus distance from the surface of the workpiece for the tool set in FIG. 5( a).

DETAILED DESCRIPTION OF THE DRAWINGS

It has been recognized that powder metallurgy manufacturing methods allow forming of internal gears to net shape, but due to residual internal porosity, powder metallurgy gears have lower strength than wrought steel gears. In the following, a process is described for the manufacture of internal gears where the net form shape capability of the powder metallurgy process is used in combination with a tooth rolling process. The tooth rolling process eliminates or reduces residual porosity hence improving the mechanical strength of the powder metallurgy gear up to the level expected from wrought steel gears. The result is the ability to produce an internal gear with economic manufacturing advantage over traditional methods because less material is wasted in the manufacturing process.

It has also been recognized that rolling an internal gear from a powder metal preform can be achieved by supporting the annulus gear against the radial forces imposed by the rolling operation. Such support facilitates tooth densification of the gear by opposing the radial forces, which would otherwise cause an increase in diameter of the annulus if the gear were not supported in this way.

Referring now to FIGS. 1( a), 1(b) and 1(c), a workpiece created using powder metallurgy for forming an internal gear 10 is supported in a work piece holding fixture 20. The internal gear 10 preferably has a set of preformed teeth 12 which are densified by internal rolling. However, it will be appreciated that the teeth may instead be formed during the rolling process. A rolling tool 30 is controlled by a spindle 40, which rotates and may feed axially (A) and radially (R) to form the internal gear of the powder metallurgy preform which may also rotate in the holding fixture at 50. As can be seen in FIG. 1( c), the gear 10 is supported about its periphery by the holding fixture 20 and the rolling tool 30 is controlled to bear against the internal surface of the gear 10 to effect roll forming thereof. As shown in FIG. 1, the support or holding fixture 20 is an annular ring that extends around the entire periphery of the preform 10. The preform 10 is a close sliding fit in the ring so as to inhibit radial expansion of the preform 10. As noted above, support of the gear 10 opposes the radial forces imparted by the rolling tool 30 to accommodate tooth densification of the gear 10 rather than allowing an increase in diameter of the gear 10.

Alternative forms of the support 20 could be used, such as a partial ring support over an arc where the tool engages the preform 10.

The spindle 40 is traversed across the complete face width of the gear 10 during the rolling operation (i.e. while rotating) and is fed into the gear 10 to the required depth for achieving the particular tooth characteristics. For example, where the gear 10 is preformed with teeth and only densification is required, the spindle 40 may be fed radially by approximately 0.1 mm, whereas for a case where the teeth are to be completely formed by the rolling operation, the spindle 40 may be fed radially by approximately 0.5 mm. The spindle 40 can be moved progressively and beneficially utilizes rate or force control as dictated by the nature of the rolling operation, the gear being produced and the choice of materials.

FIG. 2 illustrates the general displacement of material, wherein the outer diameter of the gear will remain relatively unchanged after the rolling process, but the internal gear dimensions of the major diameter, the minor diameter and the tooth thickness will be measurably changed by the forming process. Adjustment of the work piece and the rolling die dimensions can be made to affect the magnitude of change in dimensions of the work piece during the rolling process. For example working of the flanks only or combinations of flank and major diameter and minor diameter may be accomplished as desired. Preferably, the process is arranged to introduce relatively small dimensional changes to the tooth geometry during roll forming, however, as discussed above, the rolling process may alternatively be used to introduce a plain cylindrical powder metallurgy blank into the work piece fixture 20 shown in FIGS. 1( a) to 1(c) and use the forming tool 30 to completely form teeth from the original cylindrical surface.

FIG. 3 shows the desired densification of the tooth surface that is imparted during roll forming the powder metallurgy internal gear. The porosity of the preformed workpiece can be seen in the leftmost photomicrograph 3N, and the resultant densified tooth surface is shown in the rightmost photomicrograph 3D.

In FIGS. 1( a) to 1(c), the tooth form is generally straight. It will be appreciated that in other embodiments, a tooling arrangement similar to FIG. 1 can be used where the rolling tool 30 is configured to impart a final tooth form being helical rather than straight (see also embodiment shown in FIG. 5 and FIG. 6). Therefore, it can be seen that the rolling tool 30 can be adapted to impart various tooth forms as desired.

The densified layer shall have the desired characteristics as shown in FIG. 4 where in the near surface layers of less than 0.5 mm there is a shelf of essentially fully dense material, the density then reduces gradually to the core density at a distance below the surface of approximately 1 mm. The core density shown in the example of FIG. 4 is 7.0 g/cc, but depending upon the starting density of the sintered internal gear the final core density may be within the range of 6.6 g/cc to 7.4 g/cc.

In FIGS. 1 to 4, the rolling tool 30 comprises a single roller 70. Turning now to FIGS. 5( a) to 5(c), in an alternative arrangement 30 a, a plurality of rollers 72 may be employed in forming the teeth of the internal gear. It can be seen in particular in FIG. 5( c) that each of the plurality of rollers 72 is controlled and moved by respective spindles 40, which rotate and may feed axially (A) and radially (R) to form the internal gear of the powder metallurgy preform which may also rotate in the holding fixture at 50 similar to the roller 70 in FIG. 1. FIG. 6 illustrates an exploded view including one of the rollers 72. FIG. 7 shows the general displacement of material for the embodiment in FIG. 5, wherein, as above, the outer diameter of the gear will remain relatively unchanged after the rolling process, but the internal gear dimensions of the major diameter, the minor diameter and the tooth thickness will be measurably changed by the forming process. FIG. 8 shows the desired characteristics of the densified layer shown in FIG. 7.

In operation, the embodiments shown in FIGS. 1 and 5 form the internal gear 10 according to the following steps:

1) Blending a suitable powders that will provide the final desired alloy for the gear;

2) Compacting the blend preferably to around 7.0 g/cc where useful gears may also be compacted in the range of around 6.6 to 7.4 g/cc;

3) Sintering the gears as normal practice within the powder metallurgy industry;

4) If desired, coining the gears for improving dimensional accuracy;

5) Rolling the internal gears by the above-described process to impart surface density and strength to the surfaces of the gear teeth; and

6) If desired, heat treating the rolled gear to further increase strength if required by the final application.

It should be noted that the roll forming process may be beneficially applied to a wide range of powder metallurgy materials, the final alloy composition and starting density can be varied to achieve required final mechanical properties which are dictated by specific applications.

For example some gears may function adequately in the rolled condition, whereas in other applications exacting heat treatment requirements such as carburizing, carbonitriding, nitriding, nitrocarburizing or induction hardening may need to be performed.

A wide range of material carbon contents might be used. For example in a gear that would be carburized a carbon content of around 0.1 to 0.3% may be selected. For nitriding, through hardening or induction hardening applications a carbon content of 0.3 to 0.6% may be beneficial, or for gears not used in a hardening heat-treatment a carbon content of 0.5 to 1.0% may be selected.

In addition to carbon content selection, other alloying element selection would similarly be selected in view of the final application requirements. In carburizing, carbonitriding or through hardening applications, alloy content will be selected to achieve a certain level of hardenability to ensure core hardening during the quenching process. Alloying elements usually considered are Cu, Mo, Ni, Cr, Mn. In induction hardening applications lesser alloying element is usually required since no core hardening occurs during the process. In nitriding or nitrocarburizing applications higher alloy content may be selected to maintain hardness through the heat treatment process and elements that favour nitride formation may be selected.

It will be appreciated that various other applications may benefit from the above-described process and the above examples are for illustrative purposes only.

The formulated powder alloy blend will be compacted to the final internal gear shape, but with dimensional allowances that will be modified in the subsequent rolling process. The compacted piece will then be sintered using the usual processes that are used in the manufacture of powder metallurgy parts.

A coining or dimensional calibration step may or not be included after sintering depending upon the specific dimensional requirements of the part.

The internal gear rolling process is carried out on the sintered part using the tooling arrangements of the type illustrated in FIG. 1 or 5. The intention is to reduce the thickness of the sintered internal gear tooth as shown in FIG. 2 and to introduce a densified layer as shown in FIG. 3.

After the rolling process the formed gear may be heat-treated using normal methods within the gear manufacturing industry to produce the final desired mechanical properties.

It can therefore be seen that the manufacture of internal gears where the net form shape capability of the powder metallurgy process can be used in combination with a tooth rolling process for rolling an internal gear from a powder metal preform, which can be achieved by supporting the annulus gear against the radial forces imposed by the rolling operation. Such support facilitates tooth densification of the gear by opposing the radial forces, which would otherwise cause an increase in diameter of the annulus if the gear were not supported in this way. The tooth rolling process eliminates or reduces residual porosity hence improving the mechanical strength of the powder metallurgy gear up to the level expected from wrought steel gears. The result is the ability to produce an internal gear with economic manufacturing advantage over traditional methods because less material is wasted in the manufacturing process.

Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto. 

1. A method for forming an internal gear is provided comprising: providing a powder metal preform workpiece; supporting the workpiece about its periphery; and roll forming said workpiece to produce a set of densified gear teeth of a predefined dimension.
 2. The method according to claim 1 wherein said powder metal workpiece comprises a toothed preform.
 3. The method according to claim 1 wherein said powder metal workpiece comprises a non-toothed preform.
 4. The method according to claim 1 wherein said set of densified gear teeth are any one of a straight or helical form.
 5. The method according to claim 1 wherein said roll forming includes either axial or radial feeding.
 6. The method according to claim 1 wherein said roll forming includes a single rolling tool.
 7. The method according to claim 1 wherein said roll forming includes a plurality of rolling tools.
 8. A tool set for forming an internal gear comprising: a holding fixture for supporting a powder metal workpiece around the periphery of said workpiece; and a rolling tool for roll forming an inner surface of said workpiece to produce a set of densified gear teeth of a predefined dimension.
 9. The tool set according to claim 8 wherein said rolling tool is configured to apply to said set of densified gear teeth either straight or helical teeth.
 10. The tool set according to claim 8 wherein said rolling tool is fed either axially or radially.
 11. The tool set according to claim 8 comprising a single rolling tool.
 12. The tool set according to claim 8 comprising a plurality of rolling tools. 